In a blade mounting structure of a bulldozer, a structure which enables the blade to incline (hereinafter, referred to as tilting) in a left and right direction as shown in, for example, FIG. 12, is conventionally known. In FIG. 12, tip end portions of a set of left and right straight frames 3 and 3 are connected to lower portions of left and right end portions of a blade 1 via connecting members JIL and JIR, which are, for example, ball joints or cross joints (joints each having a pin turning in an up-and-down direction and a pin turning in a left and right direction), to be swingable up and down and to the left and the right. Base end portions of the set of left and right straight frames 3 and 3 are respectively connected to left and right portions of a set of left and right traveling unit frames 6 at a side of a bulldozer vehicle body via ball joints J2L and J2R such that the left and right straight frames 3 and 3 are swingable up an down and to the left and the right (See FIG. 13). A hydraulic cylinder 4 for tilt drive is connected to one of side of a left or a right side of the blade 1 between upper and lower portions of the blade 1 (in FIG. 12, the hydraulic cylinder 4 is connected the right side of the blade 1 and faces the front side of the vehicle), and a support member 4a is connected to the other side of the blade 1, between upper and lower portions of the blade 1. The hydraulic cylinder 4 and the support member 4a are also respectively connected to the set of left and right straight frames 3 and 3. The hydraulic cylinder 4 and the support member 4a are respectively connected to the sides of the blade 1 with ball joints J4R and J4L, and to the vertical surfaces of the straight frames 3 with pin connections J5R and J5L so as to be swingable. One end portion of a link member 5 is connected to a central portion of the lower portion of the blade 1 with a pin so as to be swingable up and down. Arms 2 and 2 are connected at one end portion to the set of left and right straight frames 3 and 3 and at the other end portion to the link member 5 via ball joints J3C, J3L and J3R, respectively (for example, see page 2, and FIG. 1 and FIG. 2 of Japanese Utility Model No. 2546933).
According to another conventional mounting structure of the arms 2 and 2, two arms 2a and 2a are swingably mounted on the blade mounting structure instead of the link member 5, as shown in FIG. 13. Namely, first end portions of a set of left and right arms 2a and 2a is connected to the end portions of the set of left and right straight frames 3 and 3 on the side of the blade 1 via ball joints J6L and J6R so as to be swingable up and down and to the left and the right. The second end portions of the arms 2a and 2a are swingably connected to each other via a ball joint J7. The arms 2 and 2 are swingably connected to the second end portions of the set of left and right arms 2a and 2a via ball joints J8 and J8.
However, in the above-described conventional blade mounting structure, the following problems exist. The set of left and right arms 2 and 2, or the two sets of left and right arms 2 and 2, and 2a and 2a are mounted to a rear side of the lower portion of the blade 1. However, a predetermined clearance (corresponding to a clearance d shown in FIG. 13) is needed to avoid interference between the set of left and right arms 2 and 2 and a front portion of the vehicle main body, and therefore the blade 1 has to be located away from the vehicle body. In addition, the set of left and right arms 2 and 2, or the two sets of left and right arms 2 and 2, and 2a and 2a are connected to the blade 1. Consequently, it becomes difficult to ensure visibility of the blade 1 from a driver's seat 7 provided at the vehicle body (especially, the visibility of a left portion of the blade from the driver's seat 7 which is normally provided to be set aside on a left side of the vehicle body), and workability is therefore unfavorable at the time of a ground leveling operation and the like by the blade 1.
Furthermore, since the number of arms 2, 2, 2a and 2a (or the set of left and right arms 2 and 2) is large, the number of welding spots of the ball joints and the like which are the connecting members of the arms 2, 2, 2a and 2a (or the arms 2 and 2) is large, thus causing the entire weight of the blade to be disadvantageously increased by welding for ensuring the base material strength. In addition, the welding operation is time consuming, which increases the manufacturing cost. Still further, there are many spots, such as ball joints, to be supplied with grease such, which reduces the maintainability of the structure.
Japanese Utility Model Laid-open No. 64-14251 describes a blade mounting device having a structure for solving the above-described problems. According to Japanese Utility Model Laid-open No. 64-14251, one arm 2 is provided instead of the above-described arms 2 and 2, and the arms 2a and 2a, as shown in FIG. 14 and FIG. 15. Further, one end portion of the one arm 2 is connected to a bracket 11, which is provided at a rear surface of the blade 1, via a ball joint J3C, and the other end portion of the one arm 2 is connected to a bracket 12, which is provided at either one of the set of left and right straight frames 3 and 3 (in the example shown in the drawings, the bracket 12 is proved at the straight frame 3 at the left side, and faces the front of the vehicle) via a ball joint J3L.
However, there exist the following problems in the blade mounting device described in Japanese Utility Model Laid-open No. 64-14251. As shown in FIG. 14 and FIG. 15, the front end portions of the straight frames 3 and 3 and the rear surface of the blade 1 are connected with cross joints J10L and J10R. The cross joint J10L is constituted by a vertical pin 14L and a horizontal pin 15L which are orthogonal to each other, and the cross joint J10R is constituted by a vertical pin 14R and a horizontal pin 15R which are orthogonal to each other. The ball joint J3C, which is a connecting portion of the arm 2 at the side of the blade 1, is disposed at a position shifted to the front in a plane view of the blade mounting structure with respect to a horizontal axis line 16 in the left and right direction which connects the axes of the left and right horizontal pins 15L and 15R (as shown in FIG. 14). As a result, the line connecting the ball joint J3C and the horizontal pin 15L is not parallel to the horizontal axis line 16.
A blade mounting structure is also known which is constructed by mounting the hydraulic cylinders 4 and 4 (the one at the left side is not shown) between the left and right straight frames 3 and 3 and the blade 1, so that the blade 1 is turned with respect to the horizontal axis line 16 (hereinafter, referred to as pitch) by extending and contracting the left and the right hydraulic cylinders 4 and 4 by equal strokes. To perform pitch with such a machine, the straight frame 3 at the left side shown in FIG. 14 swings around the line connecting the ball joint J3C and the left horizontal pin 15L with respect to the blade 1, while the straight frame 3 at the right side swings around the line connecting the left and right horizontal pins 15L and 15R with respect to the blade 1. As a result, the heights of the vertical movements of the left and right end portions of the blade 1 differ. Namely, the blade 1 tilts slightly without intention following the pitch motion. Accordingly, it may be difficult to operate the blade accurately into the blade attitude desired by the operator, and operability is therefore not favorable.
The present invention is directed to overcoming one or more of the above-mentioned problems.